1. Field of the invention
The invention relates to a process for separating pure monoglycerides, pure diglycerides, and/or pure triglycerides.
2. Brief Description of the Background of the Invention Including Prior Art
Monoglycerides and diglycerides are partial esters of glycerol with higher molecular fatty acids. Commercially available monoglycerides consist of mixtures of monoesters and diesters including minor proportions of triesters. As the monoesters and diesters of glycerol are edible, they are used in various fields of the foodstuff industry, pharmaceutical, and cosmetics industry because of their emulsifying, stabilizing, plasticizing, and thickening characteristics. For many purposes, particularly technical ones, the equilibrium mixture obtained on preparation of the glycerides may be used directly following separation of the non-reacted glycerol. In other fields of use, however, preparation of high-percentage monoglycerides and diglycerides is desired. Monoglycerides, prepared by molecular distillation and having a monoglyceride content of more than 90% are used mainly in the foodstuff industry (noodles, pastries, sweets and baking aids, margarine, ice cream).
Through addition of monoglyceride (up to 5% of palmitic acid/stearic acid monoglyceride of 90% purity, or 10% of palmitic acid/stearic acid mono/diglyceride), one achieves self-emulsifying characteristics of shortenings intended for baking purposes (superglycerolated shortenings).
The term "shortening" literally means making something shorter and is derived from the baking characteristics of these substances. Due to their specific structure, the monoglycerides are capable of altering the plasticizing effect of starch and gluten during preparation of the dough by inserting themselves in finely divided form into the homogeneous plasticized materials, thus breaking them up and making the dough smoother, i.e. shorter. At the same time, they facilitate the incorporation of air. As a consequence, there are obtained bakery products of increased volume and improved "shortness".
The essential components of margarine are edible fats and oils, drinking water, and emulsifiers. As emulsifiers, there may be used lecithin, egg yolks and/or monoglycerides and diglycerides of edible fatty acids. Margarine may also comprise taste and smell-improving additives (flavors), sour milk, skim milk, salt, starch syrup, citric acid and/or other edible acids, vitamins, as well as officially approved food colors (usually carotin or carotin-containing oils). Emulsifiers are mandatory adjuvants in the preparation of margarine as they render possible the formation of water-in-oil emulsions. Most commonly used are monoglycerides and vegetable lecithin as they assist each other in their emulsifying action. Used in practice are products comprising about 40% and about 90%, respectively, of monoglycerides of C.sub.16 /C.sub.18 -acids (palmitic acid, stearic acid, also in admixture with oleic acid) and, in addition thereto, 60% and 10%, respectively, of diglycerides. Common are additions of up to 0.5% and 0.25%, respectively, corresponding to about 0.2% of monoglyceride based on the fatty phase. Higher proportions of emulsifier are usually required for the preparation of low-calorie margarine.
Monoglycerides and diglycerides may be obtained by esterifying glycerol with fatty acids. By transesterification of triglycerides with glycerol or reaction of glycerol with fatty acids in the presence of catalysts, there are obtained mixtures composed of glycerol, mono-, di-, and triglycerides, and free fatty acids. A lately introduced technique is enzymatic cleavage of triglycerides. Each of the above methods produces a mixture of monoglycerides, diglycerides, and triglycerides. On esterification, for instance, there is obtained an equilibrium mixture which, after separation of the glycerol, comprises 60% of monoglycerides, 35% of diglycerides, and 5% of triglycerides. The starting mixture for esterification is chosen in such a way that monoglycerides are produced as the main product in the equilibrium mixture. The mixture is usually separated by molecular distillation. At the high temperatures within the film evaporator, a slight degree of disproportioning is encountered so that the monoglycerides or diglycerides contain small amounts of the other two esters as well as traces of free fatty acids. Moreover, the ratio of 1-monoglycerides to 2-monoglycerides is shifted in favor of the 2-monoglycerides.
Due to the disproportioning taking place at the temperatures within the film evaporator, monoglycerides having a purity above 95% cannot be produced economically by molecular distillation. There does, however, exist a wide interest in monoglycerides having a purity of 99% or more.
It is known that monoglycerides may be separated from a mixture of mono-, di-, and triglycerides with the aid of dense carbon monoxide. Required for the above method are, however, pressures of more than 350 atm. at temperatures of 40.degree. C. Moreover, even at pressures of 350 atm., loading is so low (less than 0.5%) to make economic recovery of high-percentage monoglycerides impossible.
Proposed in the German Patent application laid open DE-OS 23 40 566 is the use of acetone as an entrainer. Here, the monoglycerides as the more easily soluble components enter into the head product during countercurrent extraction. But the separating factors are relatively low so that recovery of pure monoglycerides is not interesting under economic aspects, especially as the loadings of from 0.5% to 1.5% thus achieved are quite small. Moreover, separation of the acetone entrainer from the product is involved and time-consuming.
The use of hydrocarbons as a cosolvent for a supercritical extractant such as carbon dioxide, N.sub.2 O, sulfurhexafluoride, trifluoromethane or tetrafluoromethane was proposed in German Patent reference P 38 25 248. This process does not suffer from the above disadvantages. However, cosolvent (entrainer) and supercritical component are not evenly distributed over fluid and liquid phases. Thus, on withdrawal of the products, different quantities of cosolvent and supercritical component will be removed from the extractant to be recycled. Thus, the composition of the extractant must be continuously corrected with the aid of a somewhat complex control and analyzing device.